The present invention relates to a process for the preparation of a vinyl chloride polymer latex whose particles have a mean diameter between 0.2 and 0.8 .mu.m and contain at least one organosoluble polymerization initiator, by polymerization in microsuspension of the corresponding monomer or monomers. The object of the present invention likewise is the latexes resulting therefrom, as well as their application as seeding products for the preparation in seeded microsuspension of homo and co-polymers of vinyl chloride. Since the particles of said latexes contain the entire amount of initiator required for the polymerization in seeded microsuspension, said polymerization does not necessitate any supplementary addition of polymerization initiator.
As used herein, the phrases "polymerization in microsuspension" or "fine suspension" mean a polymerization in the presence of an organosoluble initiator or initiators of at least one monomer dispersed by forcible mechanical means in an aqueous medium containing an emulsifier as stabilizer.
For the preparation of polymers of vinyl chloride by polymerization in seeded microsuspension, French Pat. No. 1,485,547 discloses the use as the seeding product of a latex whose particles contain the entire amount of initiator necessary for said polymerization.
The procedures used since then for the preparation by polymerization in microsuspension of such a latex have the following disadvantages:
(i) During the preparation of the dispersion of the monomer or monomers which is generally carried out at a temperature between 10.degree. and 30.degree. C., most frequently the formation of crusts is observed which disturb the operation of mechanical means used and by doing so lead to a latex whose mean particle diameter is not reproducible from one operation to another; and PA1 (ii) During the polymerization of the monomer or monomers thus dispersed, the large quantity of initiator present, which can be equal to more than 20 times the amount needed for the polymerization, frequently leads to sharp temperature increases of the reaction medium, which can exceed 10.degree. C., and to considerable crust formation.